Ron Wilson

Ron Wilson

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Mimosa Webworm: It’s Time for “Torched” Honeylocusts - Buggy Joe Boggs

Mimosa Webworms (Homadaula anisocentra, family Galacticidae) have been slowly developing their telltale webbed nests since late spring. However, damage by the caterpillars of this non-native moth is just now becoming evident on honeylocust (Gleditsia triacanthos) which is the webworm’s alternate host in the U.S.

The caterpillars feed gregariously as skeletonizers within webs spun over the foliage; they only feed on leaflets enveloped by their silk nests. Attention is usually drawn to an infestation by clusters of orangish-brown "torched" leaves and leaflets that are so tightly encased in webbing the foliage looks like it's melting.

Dave Shetlar (Professor Emeritus, OSU Entomology) has reported that he’s seeing heavy localized infestations on honeylocusts in central Ohio. Curtis Young (OSU Extension, Van Wert County) reported that localized damage is common in the northwest part of the state. Amy Stone (OSU Extension, Lucas County) observed pockets of mimosa webworm damage as she drove across several northeastern and northern counties late last week on her way to her home base. Likewise, I’m seeing localized torched honeylocusts in Greater Cincinnati.

“Flaming” Black Locusts Muddy the Waters

Dave also reported that he’s seeing pockets of high populations of Locust Leafminers (Odontota dorsalis) on their namesake host, black locust (Robinia pseudoacacia), in central Ohio. This native beetle is notorious for producing a captivating reddish-brown leaflet coloration on black locusts. When locust leafminer populations are high, “flamed” black locusts become a familiar sight to travelers motoring on Ohio's interstate highways.

The damage is caused by both the larvae which feed as leafminers and the adults which feed as skeletonizers. There are two generations per season with the first generation initiated by overwintered adults.

However, locust leafminers do not infest honeylocusts and mimosa webworms do not infest black locusts. It’s a reminder that a correct tree identification is a critical first step in making a correct pest identification. This is particularly important given that both black locusts and honeylocusts are commonly found growing “wild” in Ohio often near roadways.

Although there are some nice black locust selections for landscapes such as golden black locust (R. pseudoacacia 'Frisia') and R. pseudoacacia 'Purple robe', black locust trees are not generally planted in Ohio landscapes. However, there are numerous versions of honeylocusts that have long been used in landscape plantings. Indeed, Ohio is home to the first thornless honeylocust (G. triacanthos var. inermis ‘Moraine’) introduced in 1949 by the Siebenthaler Company of Dayton, OH, for use in landscapes. It was the first shade tree to ever receive a plant patent (Plant Patent 836). Consequently, it’s important to understand how to manage mimosa webworm.

A Brief History of the Silk (Tree) Road

Mimosa (a.k.a. silk tree) (Albizia julibrissin) is native to a large region from Iran to Japan. It was first brought to the U.S. from Asia as an ornamental in the mid-1700s. Unfortunately, its aggressive nature was eventually revealed with mimosa trees escaping cultivation to become naturalized in the southeastern U.S.

Mimosa webworm was accidentally introduced into the U.S. from China in the early 1940s. Contrary to some online references that claim the webworms were first found on honeylocust trees, a 1943 scientific paper described the webworm as a new pest of mimosa in the Washington, D.C. region.

Mimosa trees have a series Achille’s heel which may present a challenge to the success of mimosa webworm. The trees are highly susceptible to a vascular wilt disease caused by a pathogen specific to Albizia spp., Fusarium oxysporum forma specialis perniciosum.

Mimosa also struggles in Ohio owing to poor winter survival. Many of the mimosa trees that were doing well in the southern part of the state were wiped out during the polar vortex that flowed across Ohio during the winter of 2014-15.

Mimosa webworm continues to be found on its namesake host. However, a paper published in 1947 reported that the non-native moth had developed a taste for honeylocust. This paper also provided a hint that webworm host preferences are not equal among all honeylocusts.

Once mimosa webworms jumped ship to utilize honeylocusts, the moths used their newfound host to spread across much of the eastern and Midwestern U.S. Their spread was aided by honeylocusts becoming the go-to tree to replace American elms (Ulmus americana) killed by Dutch elm disease.

Digging Deeper (Into Silk Nests)

Mimosa webworms have three generations per season in Ohio. This was confirmed last season by Dave Shetlar’s light trap catches and other observations. This is important because populations expand with each successive generation, so the more generations, the greater the seasonal damage. 

Also, nests become larger and more obvious with each generation. Research published in 1993 revealed that the caterpillars dispense a water-soluble chemical onto the webbing that stimulates female moths to lay eggs. So, females lay their silverish-white eggs on the nests from which they developed. New eggs are silverish-white and turn coral-red as they age.

Consequently, first-generation nests are expanded by second and third-generation caterpillars. This partially explains why the moths commonly fly below our radar until leaves turn brown as nests are enlarged by the second and third-generation caterpillars.

However, webworm development is always synchronized. The generations may slightly overlap meaning that it's common to find relatively large caterpillars in nests containing small caterpillars. This is particularly true between the second and third generations.

First and second-generation caterpillars pupate in the nests with moths emerging from the nests. Third-generation caterpillars vacate the nests by making controlled descents on silk threads so they can pupate in the soil. First, second, and third-generation caterpillars may also leave their nests if they deplete their food supply.

Either way, rappelling caterpillars become repelling if they drop onto unsuspecting picnickers or into associated food and beverages (e.g., mimosa cocktails). They can become a serious nuisance pest around backyard swimming pools where honeylocusts have long been a favored tree owing to their filtered shade, good branch structure, and small leaflets that minimize fall pool maintenance.

A Word from Management

Relying on host range is a highly effective first step in managing any tree pest. There’s no need to consider any other management options if the pest will not feed on a tree.

Research has revealed that there are distinct differences in terms of host suitability among the thornless honeylocusts introduced since 'Moraine'. A paper published in 1990 showed females reared on 'Moraine' produced significantly fewer eggs compared to females reared on 'Imperial', 'Shademaster', 'Sunburst', and 'Skyline.'

However, this leads to two questions drawn from rampant speculation. First, is the highly localized nature of mimosa webworm outbreaks based on chance, or is it associated with a localized abundance of more susceptible cultivars? Second, has the genetic resistance against mimosa webworm been reduced or lost as honeylocusts were selected for preferred horticultural traits?

The environment also plays a key role in mimosa webworm population dynamics. Like their namesake host tree, overwintering mimosa webworm pupae have a low-temperature Achilles' heel. The 2014-15 winter polar vortex had a serious impact on the winter survival of mimosa webworm.  In fact, last year was the first season since the calamitous polar express that we saw widespread webworm damage. It appears the relatively mild winters recently enjoyed by Ohioans are allowing localized webworm populations to continue to build in some parts of the state.

However, given that localized population trajectories tend to slowly rise year after year until peaking in an "outbreak" and then collapsing, the depredations of mimosa webworms alone are not considered sufficient to kill established trees. Mimosa webworms are generally considered an aesthetic as well as a nuisance pest problem on healthy, established trees. The nests make trees unsightly, and caterpillars will occasionally drop from infested trees onto unsuspecting backyard gardeners, grill masters, dog walkers, etc.

Although the mimosa webworm moth is a non-native, this exotic pest has been with us long enough to become targeted by predators, parasitoids, and pathogens (the 3-Ps). A paper published in the Great Lake Entomologist in 1987 reported nine parasitoids including both flies and wasps were recovered from overwintering pupae. A study conducted in Ames, IA, and published in 1990 found parasitism rates by the wasp, Elasmus albizziae, on first-generation mimosa webworm pre-pupae to range from 44% to 47% over three consecutive years.

Indeed, the picture below shows a parasitoid wasp I found cavorting among early instar mimosa webworms in Wyoming, OH. Its antlered antennae indicate this wasp belongs to the Family Eulophidae. Wasps in this family are ectoparasites meaning they lay their eggs on the surface of their victims. The resulting wasp larvae bore a hole through the integument to zip in and out as they consume the victim's innards.

I took the following picture of a potter wasp (Parancistrocerus leionotus, family Vespidae) grabbing webworm caterpillars to provision their young. Potter wasps are so named for creating pot-like mud structures; however, this species only uses mud to fashion chambers in rock crevices.

While healthy honeylocust trees can recover from occasional mimosa webworm outbreaks, the impact may be different for newly planted trees as well as older trees planted in confined spaces such as in "tree wells" or between streets and sidewalks; the so-called "devil's strip." The added stress may push the trees over the edge or make them susceptible to opportunistic borers such as the honeylocust borer (Agrilus difficilis). This is particularly true if webworm outbreaks occur during a drought year.

Insecticide applications may be required to protect vulnerable trees. However, topical applications are not generally recommended for two reasons. First, they will kill the bio-allies such as the aforementioned parasitoid wasps that provide natural control of mimosa webworm. Second, dense webworm nests present a significant barrier to insecticide penetration. This is particularly true for second and third-generation nests. 

If insecticides are required, systemic insecticides are the best option because there is a much-reduced chance they will kill the beneficial insects. Of course, if the goal is to reduce leaf loss on vulnerable trees, the application timing should target first-generation caterpillars. Also, preventing first-generation nests from fully developing will reduce the attraction of trees to second and third-generation female moths.

The systemic neonicotinoids clothianidin (e.g. Arena 50WDG), dinotefuran (e.g. Safari, Transect, etc.), and acetamiprid (e.g. TriStar) are effective against these caterpillars. Applications should follow label directions relative to soil drench or trunk sprays. Acephate (e.g. Lepitect or Lepitect Infusible) applied as soil drenches or trunk injections are also effective.

It appears we are largely transitioning from second to third-generation webworms in southern Ohio. This means if the overarching goal is to protect vulnerable trees by preventing leaf loss, the battle is rapidly becoming lost. Arguably, using an insecticide this late in the game would mostly be a "feel good" application. Tree care professionals should note the infestations in the records they keep on their clients so trees can be closely monitored next year to target first-generation webworms. After all, a localized outbreak this season does not necessarily mean a repeat next season.

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